Many satellites and other spacecraft, as well as some terrestrial stationary and vehicle applications, such as seagoing vessels, include electrical distribution systems that have both a primary electrical power source and a backup electrical power source. For example, many satellites include an array of solar cells, which may be attached to the satellite itself or to solar panels that extend from the satellite. The solar arrays are typically the primary electrical power source whenever the satellite is exposed to the sun. However, a satellite can experience periodic eclipses from the sun when the satellite's orbit moves it into the Earth's shadow. During these eclipse periods, a backup power source is used to supply electrical power. In addition, while the solar arrays may be sized to handle at least nominal design power loads, the arrays may not be sized to handle transient and/or peak design power loads. Thus, the backup power source may also be used during periods of transient and/or peak power demands to augment the solar arrays.
In some spacecraft systems, the backup power source is one or more rechargeable batteries. During eclipse periods, transients, and/or peak power demand periods, the batteries supply some or all of the electrical power to the satellite's electrical distribution system, causing the batteries to discharge. Thereafter, the batteries are recharged to capacity using excess electrical energy from the solar arrays. The lifetime of many batteries is limited according to a number of charge-discharge cycles. In addition, some batteries are relatively heavy. Thus, some satellites plan to include one or more energy storage flywheel systems to either supplement or replace batteries as the backup power source. In some satellite concepts, energy storage flywheel systems are used as both a backup power source, and to supply attitude control for the satellite.
Energy can be stored in various forms, including as electrical energy or as mechanical kinetic energy. Energy storage flywheel systems may be thought of as “mechanical batteries,” that convert electrical energy into rotational kinetic energy, and rotational kinetic energy into electrical energy. Energy storage flywheel systems can include one or more flywheels that are rotationally mounted using magnetic bearings, and that are coupled to a motor/generator and, if also used for attitude control, may be coupled to a gimbal actuator. To convert electrical energy to rotational kinetic energy, the motor/generator is operated in a motor mode and is used to rotate the flywheel up to a relatively high rotational speed. To convert the stored kinetic energy to electrical energy, the motor/generator is operated in a generator mode and is rotated by the flywheel to generate electrical energy.
The operation of each motor/generator is preferably controlled in such a manner as to provide stable power bus voltage. However, in many instances the motor/generators are controlled using individual controllers that implement fairly simple control schemes. This can result in unbalanced load sharing among the flywheels and other components in the system, which can in turn result in speed mismatches, undesirable heating, reduced reliability, and circuit and component failures.
Hence, there is a need for a system and method of controlling energy storage flywheel system motor/generators that regulates system voltage, while providing balanced load sharing amongst a plurality of flywheel systems, and/or reduces undesirable heating, and/or increases system reliability, and/or reduces the likelihood of circuit and component failures. The present invention addresses one or more of these needs.